1. Field of the Invention
The invention relates to the purification of a hydrogen-containing gas stream. More particularly, it relates to the use of a pressure swing adsorption system for the recovery of a purified hydrogen-containing product.
2. Description of the Prior Art
More than 90% of all the hydrogen presently used is produced either by steam reforming or by partial oxidation of various hydrocarbons. The effluent from such operations, or hydrogen-containing effluent gas streams from other operations such as coal gasification, are generally subject to the well-known water gas shift reaction to convert carbon monoxide therein to additional hydrogen and to carbon dioxide. After high temperature shift, the effluent gas streams typically have compositions, in mol % on a dry basis, on the order of the following:
______________________________________ CO, CH4 Hydrogen CO.sub.2 and other ______________________________________ Steam Reforming Effluent 68-75 17-25 2-15 Partial Oxidation or 60-65 30-35 3-8 Coal Gasification Effluent ______________________________________
The thus-shifted gas stream is then further treated by alternative procedures to obtain the desired hydrogen product. Thus, the gas stream may be passed to a scrubbing unit for the removal of CO.sub.2 down to about 0.1%. The effluent therefrom can be further treated to remove additional CO.sub.2 so as to reduce the residual content thereof to about 1 ppm. Following such carbon dioxide removal, the effluent gas stream can be subjected to liquid nitrogen scrubbing or to cryogenic purification techniques to produce about a 97% hydrogen product stream, or a pure ammonia syngas stream, together with by-product fuel gas.
Alternatively, the effluent from high temperature shift conversion can be subjected to low temperature shift conversion for further CO conversion, and then passed to a scrubbing unit to achieve CO.sub.2 removal down to about 0.1%. The effluent therefrom can then be subjected to methanation to produce said 97% hydrogen product stream. By either approach, essentially complete removal of carbon dioxide from the effluent stream is required, and in the case of the latter approach, both high temperature and the more costly, and more sensitive, low temperature shift conversion must be employed.
The desired hydrogen purification can also be accomplished by means of a process and system utilizing pressure swing adsorption (PSA) for final purification. Such a PSA approach has the advantages of simplification, ease of operation and of high hydrogen product purity, i.e. 99.99+%, than is achieved by the alternative approaches referred to above. There is a genuine desire in the art that improvements be developed to enhance the feasibility of employing the PSA approach in practical commercial operations. In particular, the obtaining of improvements in the recovery of purified hydrogen product is desired so that the size of the stream reformer, partial oxidation or other hydrogen-containing effluent gas stream generation unit can be optimized.
It is an object of the invention, therefore, to provide an improved process and system for the production of hydrogen-containing product gas.
It is another object of the invention to provide a process and system for the enhanced recovery of purified hydrogen product from effluent gas streams.
It is a further object of the invention to provide an improved overall process and system for the treatment of an effluent gas stream in which final purification of a hydrogen-containing product gas stream is achieved by pressure swing adsorption at high purity and recovery levels.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.